1. Field of the Invention
This invention relates to an information readout apparatus and an information readout method for reading out information from a recording medium such as an optical disk in a controlled manner. More particularly, the present invention relates to an information readout apparatus and an information readout method adapted to optimally control the detection window of a super resolution optical disk that is operating as readout area within the light beam irradiation range of the disk.
2. Prior Art
At present, optical disks are popularly used as medium for storing image information, sound information and computer data because of the advantage of having a large storage capacity.
In recent years, various technologies have been developed for raising the signal recording density of recording media such as opto-magnetic disks and phase change disks. Particularly, technologies of super resolution readout for reading out a recorded mark smaller than the diameter of a light spot formed by a light beam have been drawing attention.
Known super resolution disks include MSR (magnetic super resolution) disks, PSR (phase change super resolution) disks and RSR (ROM super resolution) disks. On the other hand, known readout systems include the RAD (rear aperture detection) system, the FAD (front aperture detection) system and the double mask system which are discriminated from each other in terms of the position of the readout area to be detected by means of a light spot formed by a beam spot. The CAD (central aperture detection) system is also known as a variation of the RAD system.
As disclosed in a number of patent documents of the applicant of the present patent application including Japanese Patent Applications Laid-Open Nos. 3-93056 and 3-93058, an MSR disk of the RAD system represents the technology of forming a recording layer by using a multilayer film comprising a readout layer and a recording/holding layer that are magnetically coupled with each other and heating the readout layer to a predetermined temperature range by irradiating it with a laser beam for the purpose of readout so that only the magnetized signals stored in the heated area of the recording/holding layer can be transferred to the readout layer and actually read out. In other words, a recorded mark smaller than the diameter of the light spot formed by the laser beam can be transferred and read out by means of this technology. The area from which the magnetized signals are transferred to the readout layer and can be read out is referred to as detection window or aperture.
On the other hand, an MSR disk of the CAD system is so adapted that an opto-magnetic recording medium comprising a recording layer and an intra-planarly magnetizable readout layer is irradiated with a light beam from the side of the readout layer to raise the temperature of the readout layer above a predetermined level only in the area irradiated with the light beam. Then, the state of magnetization of the readout layer is shifted from that of intra-planarly magnetization of only the detection window (aperture) in the area that is irradiated with the light beam to raise the temperature thereof above the predetermined level to that of vertical magnetization of transferring the magnetism of the detection window to the corresponding area of the recording layer. As a result, a recorded mark smaller than the diameter of the light spot formed by the light beam can be read out.
Thus, with the MSR technology, a super resolution readout effect is realized as the data written on the recording layer is magneto-optically read out while shifting the state of magnetization of the readout layer.
As for data readout using the MSR (magnetic super resolution) technology, it is known that the readout condition for optimizing the signal quality varies as a function of the sensitivity of the medium, the ambient temperature and perturbation such as skew of the medium substrate.
Meanwhile, with the RAD system, the size of the aperture varies as a function of the readout power of the light beam. Therefore, as the readout power is raised, the size of the aperture increases to consequently increases the area to be used for reading out the recorded mark on the recording layer arranged under the readout layer. With this system, then, while the C/N ratio (the carrier to noise ratio) rises, the super resolution effect is reduced so that any of the recorded marks of the adjacent tracks may be read out and hence unrelated signals may become detected to raise the cross talk level. On the other hand, the C/N ratio lowers as the readout power is reduced.
As pointed out above, the inter-code interference factor and the cross talk factor increases when the readout power is too strong, whereas the C/R ratio falls when the readout power is too weak. In any case, the jitter that appears as positional error of changeover from 1 to 0 or vice versa becomes remarkable when reading out the signal.
In view of this problem, Japanese Patent Application Laid-Open No. 8-63817 proposes a technique of optimizing the effect of signal readout by detecting the amplitude of each of the read out signals representing so many patterns of recorded marks having different lengths, comparing the signal levels and controlling the readout power to bring them close to a reference value. With this technique, for instance, the size of the detection window can be controlled by reducing the readout power when the ambient temperature is high.
However, with this known technique, the signals representing the respective patterns of recorded marks for detecting the resolution are arranged on the data area of the optical disk to consequently give rise to a problem of an enhanced level of redundancy, which by turn reduces the available recording capacity of the optical disk. Additionally, if a specific area (lead-in area) is provided on the optical disk to store the signals representing the respective patterns of recorded marks, a track jumping action is needed to get to that specific area in order to control the readout power during a signal readout operation at the cost of the access efficiency of the optical disk. If, on the other hand, the signals representing the recorded patterns are distributed over the entire surface of the optical disk to avoid this problem, they are more affected by the surface defects, if any, of the optical disk.
Still additionally, there is a problem that the amplitudes of the signals cannot be sampled at the peak level depending on the modulation system if the readout clock for demodulating data is used for the sampling. Then, it may be totally impossible to detect the amplitudes of the signals.
In view of the above identified problems, it is therefore the object of the present invention to provide an information readout apparatus and an information readout method adapted to efficiently correct any change in the aperture size without sacrificing the recording capacity and without using a clock dedicated to detect the amplitude of the read out signals having respective specific mark lengths.